1. Field of the Art
The present invention relates to electric motors, and more particularly to a fabrication method and related assembly of potted electrical motors.
2. Discussion of the Related Art
U.S. Pat. No. 4,922,604 to Marshall et al. and U.S. Pat. No. 5,008,572 to Marshall et al., both assigned to an assignee of the present invention and hereby incorporated by reference, describe an electrical motor and fabrication method for achieving precise bearing registration of an internally disposed rotor assembly. As described more fully in the aforementioned references, an electrical motor comprising stator and rotor assemblies is fabricated by utilizing a potting compound to both unitize the stator assembly and provide improved thermal characteristics.
More particularly, the stator assembly is formed from a stack of stator laminations having inwardly projecting poles that define the walls of an unfinished internal chamber for housing a rotor assembly. Through- bolts attach front and rear aluminum end caps to the lamination stack, and this entire stator assembly is potted to unitize the lamination stack and fill voids between the stator poles with potting compound. The internal chamber is then finished by machining a continuous, cylindrical bore substantially coincident with the sidewalls defined by the inwardly projecting stator poles. Precision registration of the rotor assembly is achieved by aligning end bearings on the rotor shaft with internal receiving surfaces machined in the stator end caps; the receiving surfaces being part of the cylindrical bore.
While the motor and fabrication method described in the '604 and '572 references achieve substantial advances over the prior art, further improvements are still desired. In this regard, various features of the assembly and method described in the '604 and '572 references were identified for improvement.
One area sought to be improved relates to the potting of the stator assembly. Specifically, it has been determined empirically that a double cure cycle is desired for the potting compound; the second cure cycle improves the rigidity of the stator assembly, thereby yielding more efficient operation over the life of the motor. When only a single cure cycle is employed it has been found that, after a period of use, the rotor assembly occasionally locks within the stator assembly, thereby limiting the useful life of the motor. Adding a second cure cycle, however, undesirably increases manufacturing costs.
A related aspect sought to be improved is the removal of air pockets formed within the potting compound. As discussed in the '572 and '604 references, air is a poor thermal conductor. Filling the otherwise open space between the stator poles with potting compound, improves thermal conductivity and, accordingly, improves dissipation of the heat developed within the motor. The process used in potting the stator assembly, however, is known to leave an air pocket under the rear end cap. This impedes the flow of heat from the rear end cap. Therefore, further improvements in the thermal characteristics of the motor assembly are sought by eliminating the air pocket.
Another area sought to be further improved relates to providing external electrical connections to the stator windings within the motor assembly. As described in the '572 and '604 references, an insulator having connectors is seated against the rear end of the stator lamination stack. The terminating ends of the stator windings are brought out to the connectors, and a printed circuit board is configured to plug into connectors, establishing electrical continuity between the stator windings and conductive traces on the printed circuit. A second connector having conductive pins electrically connected to the conductive traces on the printed circuit, is provided in conjunction with the printed circuit board to accommodate remote access to the stator windings by way of a complementary external connector. In this way, external signals are readily connected to the stator windings for controlling the motor operation.
This second connector is nested within the rear stator end cap. As previously described, both end caps are attached to the lamination stack by means of several through bolts, before the potting compound is applied. To prevent any potting compound from leaking between the rear end cap and the second connector, a sealing means is provided around the second connector. Although the aforementioned structure provides an effective means for connecting external motor drive signals to the stator windings, a simpler and therefore more cost-effective solution is desired.
In short, improvements are sought which will more efficiently address the aspects described above, while maintaining the benefits and advantages in the motor assembly and method as described in the '572 and '604 references.